Workaround backward compatibility issues with older setuptools versions due to license-files

The license-files field in the pyproject.toml was not allowed in
older setuptools versions:

ValueError: invalid pyproject.toml config: `project`.
configuration error: `project` must not contain {'license-files'} properties

This commit worksaround the issue by moving the license-files back
to setup.py.

Author: lxp

pyproject.toml

@@ -12,7 +12,6 @@ package-dir = {"" = "lib"}
 name = "cfv"
 description = "Command-line File Verify - versatile file checksum creator and verifier"
 readme = "README.md"
-license-files = ["COPYING", "lib/cfv/BitTorrent/LICENSE.txt"]
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",

setup.py

@@ -31,4 +31,5 @@ def _get_version(path):
     # backward compatibility issues with older setuptools versions.
     # See: https://github.com/pypa/setuptools/issues/4903
     license_expression='GPL-2.0-or-later AND MIT',
+    license_files=('COPYING', 'lib/cfv/BitTorrent/LICENSE.txt'),
 )